The present invention relates, in general, to the bending of sheets of material, and more particularly, relates to slitting of the sheet material in order to enable precision bending.
A commonly encountered problem in connection with bending sheet material is that the locations of the bends are difficult to control because of bending tolerance variations and the accumulation of tolerance errors. For example, in the formation of the housings for electronics, sheet metal is bent along a first bend line within certain tolerances. The second bend, however, works off of the first bend and accordingly the tolerance errors accumulate. Since there can be three or more bends which are involved to create an enclosure, the effect of cumulative tolerance errors in bending can be significant.
One approach to this problem is to try to control the location of bends in sheet material through the use of slitting. Slits can be formed in sheet stock very precisely, for example, by the use of computer numerically controlled (CNC) controllers which control a slitter, such as a laser, water jet or punch press. Referring to FIG. 1, a sheet of material 21 is shown which has a plurality of slits 23 aligned in end-to-end, spaced apart relation along a proposed bend line 25.
Between pairs of slits are bending webs 27 which will be plastically deformed upon bending of sheet 21 and yet hold the sheet together as a single member.
The location of slits 23 in sheet 21 can be precisely controlled so as to position the slits on bend line 25 within relatively close tolerances. Accordingly, when sheet 21 is bent after the slitting process, the bend occurs at a position that is very close to bend line 25. Since slits can be laid out on a flat sheet of material precisely, the cumulative error is much less in such a slitting-based bending process as compared to one in which bends occur in a press brake with each subsequent bend being positioned by reference to the preceding bend.
Nevertheless, even slitting-based bending of sheet material has its problems. First, the stresses in bending webs 27, as a result of plastic deformation and slitting at both ends of webs 27, are concentrated. Thus, failures at webs 27 can occur. Moreover, the slits do not necessarily produce bending of webs 27 directly along bend line 25. Thus, in prior art slitting processes the problem of cumulative error in the bend location has been reduced, but stress concentration and somewhat erratic bending can occur.
Accordingly, it is an object of the present invention to provide method for precision bending of sheets of material using improved slitting techniques which both reduce stress concentrations at the bend web and enhance the accuracy of the bends.
Another object of the present invention is to provide a precision sheet bending process and a sheet of material which has been slit for bending and which can be used to accommodate bending of sheets of various thicknesses and of various types of materials.
A further object of the present invention is to provide a sheet bending method which results in a bent product having improved shear loading capacity.
Another object of the present invention is to provide an method for slitting sheets for subsequent bending, and the sheets themselves, that will accommodate both press brake bend and slit bends, is adaptable for use with existing slitting devices, enables sheet stock to be shipped in a flat condition and precision bent at a remote location without the use of a press brake, and enhances assembly or mounting of components in the interior of enclosures formed by bending of the sheet stock.
The method for precision bending of sheet material, and the sheet stock formed for such precision bending, of the present invention has other features and objects of advantage which will become apparent from, or are set forth in more detail in, the accompanying drawing and the following description of the Best Mode of Carrying Out The Invention.
In one aspect, the method for precision bending of a sheet of material of the present invention is comprised, briefly, of the steps of forming a plurality of longitudinally extending slits through the sheet in axially spaced relation in a direction extending along, and proximate to, a bend line to define bending webs between adjacent ends of pairs of the slits; and forming a stress reducing structure at each of the adjacent ends of the pairs of slits. The stress reducing structure can be provided by openings or transversely extending, preferably arcuate, slits formed on the bend line and opening to the longitudinally extending slits. The stress reducing openings have a transverse width dimension which is substantially greater than the transverse width dimension of the longitudinal slits, and the arcuate stress reducing slits are convex in a direction facing the bending webs. A further step of the method is the step of bending the sheet material substantially along the bend line across the bending webs between the stress reducing structures.
In another aspect, the method of the present invention includes slitting a sheet of material for precision bending which comprises the steps of forming a first elongated slit through the sheet of material along the bend line by forming a pair of proximate, transversely spaced apart, parallel and longitudinally extending, first slit segments connected near a common transverse plane by a transversely extending slit segment; and forming a second elongated slit in substantially longitudinally aligned and longitudinally spaced relation to the first elongated slit. The step of forming the second elongated slit also preferably is accomplished by forming a pair of proximate, transversely spaced apart, parallel and longitudinally extending, slit segments connected near a common transverse plane by a transversely extending slit segment. Thus, instead of one continuous elongated slit, each slit in the pair of slits is formed as a slightly stepped slit proximate a midpoint of the combined length of the slit segments. This structure produces a virtual fulcrum upon bending that can be positioned precisely on the bend line to cause bending of the bending webs more precisely along the bend line. In the most preferred form, the stepped slits are also provided with enlarged end openings so as to reduce stress concentrations at the bending webs.
The present invention also includes a sheet of material formed for precision bending comprising a sheet having elongated slits which are spaced apart in end-to-end relation and in substantial alignment along the bend line, and stress reducing structures at the ends of the slits to reduce stress concentrations. In the most preferred form the sheet of material further has the slits formed as stepped slits in which proximate, transversely spaced apart, parallel and longitudinally extending, slit segments are connected proximate a transverse intermediate plane by a transversely extending slit segment so that bending occurs at a virtual fulcrum. During bending, between the longitudinally extending slit segments tabs formed by the stepped slits slide on supporting edges of the sheet positioned across the slits from the tabs.